1. Technical Field
The present invention relates to electroluminescent (this is herein abbreviated to “EL”) phosphor powder that comprises zinc sulfide as the matrix thereof and contains both an activator and a co-activator to be the radiative recombination center thereof, and to a bright and long-life EL device that comprises the powder.
2. Background Art
An EL device emits light when excited by electric power, for which known are a dispersion-type EL element where phosphor powder is sandwiched between electrodes, and a thin film-type EL element. Generally, a dispersion-type EL device is so designed that a dispersion of phosphor powder in a binder having a high dielectric constant is sandwiched between two electrode sheets at least one of which is transparent, and this emits light when an alternating current is applied between the two electrodes. The EL device that comprises such EL phosphor powder has many advantages in that it may be thinned to have a thickness of a few mm or less and, since it is a surface-emitting device, it does not generate heat and its light emission efficiency is high. Therefore, EL devices are expected to have many applications for traffic sings, lighting equipment for various interiors and exteriors, light sources for flat panel displays such as liquid-crystalline displays, lighting equipment for large-area advertising pillars, etc.
EL phosphor powder well known in the art comprises zinc sulfide as the matrix thereof, along with an activator such as copper (metal ion serving as a radiative combination center) and a co-activator such as chlorine added thereto. However, the light-emitting device that comprises the phosphor powder has some drawbacks in that its brightness is low and its light emission life is short, as compared with those of light-emitting devices based on any other principle, and therefore, various improvements have heretofore been made on the phosphor powder.
Regarding the structure of phosphor particles that enable light emission of high brightness, JP-A 8-183954 (pp. 3–4, FIG. 1) discloses zinc sulfide phosphor particles that are characterized in that they have real stacking defects of high density uniformly and everywhere in each particle and the mean spacing of the stacking defects is from 0.2 to 10 nm. This says as follows: In the particles, copper ions serving as an activator are localized in the stacking defects of the matrix crystal of zinc sulfide, and they form conductive layers. Accordingly, when a voltage is applied thereto, the particles may release electrons and holes at high efficiency, and therefore enable light emission of high brightness.
On the other hand, using a single crystal of zinc sulfide, the relationship between the light-emitting mechanism and the structure of the crystal particles has been studied in detail. In particular, an important conclusion has been obtained for the relationship between the direction of the electric field applied to them and the orientation of the phosphor particles (Physical Review 149–158, Vol. 125, No. 1 (1962), page 150, FIG. 1). Specifically, when the direction of the electric field applied to them and the (111) face of the zinc sulfide phosphor particle are parallel to each other, then the brightness of light from the particles is the maximum.
When single-crystal zinc sulfide is used, the direction of the electric filed to be applied to it and the crystal orientation may be controlled. However, in a dispersion of fine particles, the individual phosphor particles are randomly dispersed, and the dispersion will be applied to a substrate by printing (or coating) thereon. In this case, therefore, when an electric field is applied to them, the individual phosphor particles are randomly oriented relative to the electric field, and, as a result, only a part of the particles could emit light at high efficiency.
Taking the prior art problems into consideration, the present invention is to provide an EL device of high brightness sufficient for light emission and to provide an EL phosphor powder for it.